Chromium plating



Patented June 22, 1926.

UNITED STATES 1,589,988 PATENT orFicE- KEVIE WALDEMAR SCHWARTZ, OF NEW YORK, N. Y., ASSIGNOR, BY MESNE ASSIGN- MEN'IS, TO CHROMIUM PRODUCTS CORPORATION, OF WILMINGTON, DELAWARE, A

CORPORATION OF DELAWARE.

CHROMIUM PLATING.

N Drawing.

This invention relates to electro-deposition of chromium; and it comprises a method of electro-depositing chromium metal as a smooth, clean deposit wherein electrolysis is performed in a solution or bath containing chromic acid or chromium trioxid, a chromium salt and chromium hydroxid, with the use of a metallic mass of chromium or metal containing chromium as a soluble 1 anode to replenish the bath; all as more fully hereinafter set forth and as claimed. Chromium is a metal of many peculiar properties, some of which render it inghly desirable in the arts. For one thing, it is not highly corrodible and retains a'brilliant surface for long periods. Because of these properties, it is a desideratum to produce it as an electro-deposited coatingfor other and cheaper metals. Such an electro-deposition 0 is practicable by methods outlined in the patent to Grube 1,496,845 wherein is used a bath containing a chromium salt, such as chromium sulfate, chromic acid or chromium trioxid, and a certain amount of chromie hydrate or hydrated sesquioxid, Cr(OH) The theory of the utility of this solution in the electro-deposition of chromium is not definitely worked out; but its properties-may depend upon the formation of some chromate of chromium, i. e., a combination be tween CrO which is an acid body, and (X 0 which is a basic body, in some proportion. Electrolysis is performed with insoluble anodes, which may be of lead peroxid, and the chromium content of the bath is replenished from time to time by the addition of (k0,.

Among other peculiarities of chromium is that of the ease with which it assumes a passive condition in contact with various oxidizing solutions; a condition in which it is not attacked by acids or the usual agents which cause corrosion. This is, of course, one of the reasons for the desirability of its use as a coating metal. Because of this passivity in oxidizing solutions generally usually thought that neither metallic chromium nor alloys containing chromium, such as ferro-chromium, can be used as a soluble anode in any bath of oxidizing and acid character.

Nevertheless, and rather unexpectedly, I have found that metallic chromium and.

Application filed November 11, 1924'. Serial No. 749,347.

metals containing chromium may be used satisfactorily as soluble' anodes in chromic acid baths of the type of that in the acknowledged Grube patent, dissolving regularly and satisfactorily. Ordinarily in the use of such a bath there are employed insoluble anodes (lead peroxid, etc.) and replenishment is by the use of soluble CrO added from timeto time. By the use of anodes made of chromium or ferro-chromium there is a better operation of the bath as a whole, since the composition is maintained more constant than is practicable by the addition of CrU as a replenishing material. Chromium or ferro-chromium anodes may be used to give all the chromium required for replenishment or any part of it. It is possible to use chromium or ferrochromium anodes as, so to speak, stabilizing agents in Working the Grube bath in the usual way, that is with replenishment with C10,. With the anodes or some portion of them of chromium the composition of the bath is kept constant, the effect of irregularities in additions of (310, being evened out.

In a practical embodiment of the present process, an electrolyte may first be prepared by dissolving the various ingredients in a lead-lined tank fitted with a lead heating coil. Satisfactory operation will be given by a bath containing about 415 grams of commercial @1 0 about 12 grams of commercial Cr(Oll-l) and 21 grams of neutral commercial chromium sulfate in the liter. It is best to dissolve the CrO first, heating the solution to boiling and then add the chromium hydroxid while the solution is hot. When this is completely dissolved, the

chromium sulfate is added and stirred into solution. The solution so formed may serve as a stock solution for making a bath, be ing diluted somewhat for this purpose.

Electrolysis of such a bath is performed with chromium or ferro-chromium anodes of any suitable size and shape, electrolysis and especially those of acid nature, it is b eing at a rather warm temperature, advantageously between 40 and 45. G. However, the temperature may go as-high as 80 C. A voltage of from 4.5 to 6 is satisfactory. The current density at the cathode should usually range from 0.15 to 0.175 amperes per square centimeter.

' The anodes may be of pure carbon-free chromium, sucli'as is made by various elec trio furnace processes, or by the thermite process. The metal maybe cast into plates or other shapes of a size depending upon the size of the-bath and of the articles on which metallic chromium is to be deposited. It is sometimes advantageous to add some quantity of alloying metals, such as lead, cobalt and nickel to the chromium to reduce brittleness and increase the life of anodes during handling. The mixture so made may be an actual alloy.

in a particular manner in plating with chromium, using a higher voltage for the initial coating of the object to be plated; that is, for producing a flash coating, and then continuing the electrolysis at a lower voltage. For example, it is sometimes desirable to produce the initial coating with chromium at a voltage ranging between 8 and 12 Volts,

this being continued for a very short time,

merelythe time necessary to cover the complete surface to be plated. When the chromium appears on all parts of the cathode,

' the voltage is reduced to between 3 and 6.5

volts and electrolysis continued at this reduced voltage until the desired thickness of metal has been built up.

It is particularly desirable to effect vigorous agitation or stirring of the bath during electrolysis. Mechanical stirrers or gas stirring may be used. By stirring, a unlform concentration of electrolyte around the cathode and anode can be maintained during electrolysis.

Instead of using plates or shapes of chromium as the anodes it is practicable to use granular chromium or other form of granular or non-reguline metal. The material may be supported in place in any convenient and well known manner as by using bags or cages. Or the subdivided chromium may be agglomerated into shape by the use of other metals, such as lead. Alloys of chromium may be used. I regard my discovery as covering any use of chromium as the soluble anode in acid chromium depositing baths containing CrO as the main electrolyzable component.

What I claim is:

1. In the deposition of metallic chromium from aqueous baths of acid nature containing' CrO as the predominant acid, the process which comprises electrolyzing with an anode containing metallic chromium.

2. In the electrodeposition of chromium from aqueous baths of acid nature the process which comprises depositing metallic chromium from a bath containing chromium trioXid, chromium hydroxid and a soluble chromium salt, the chromium hydroXid being equal to at least twice the total amount of chromium hydroxid and chromium salt and electrolysis being performed with anodes containing metallic chromium.

3. In the electrodeposition of chromium from aqueous baths containing GrO as the main electrolyzable component and replenished with GrO the process of cheapening the operation and of maintaining constancy of composition which comprises performing electrolysis with anodes containing metallic chromium.

In testimony whereof I atfix my signature.

KEVIE WALDEMAR SCHWARTZ.

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